Passive solar heating systems featuring a series of heat tubes enclosed in a glass panel are well established in the art. One of the latest patents to be issued is that of Hunter, Modular Passive Solar Heating Systems, U.S. Pat. No. 4,505,261, dated 3/19/85. This system is representative of the flat-plate solar collectors which feature a series of heat tubes mounted in an elongate panel similar to that of the subject disclosure.